Clinical and experimental evidence suggests that two major central systems play important roles in pain sensory mechanisms: 1) direct spinothalamic projections to posterior lateral (PO) and medial-intralaminar (ML-IL) thalamus nd 2) reticulothalamic projections from nucleus gigantocellularis (NGC) of medulla. Accordingly, the PO and M-IL thalamus and NGC will be sampled, in the anesthetized squirrel monkey, for neurons responding exclusively or differentially to noxious heat. Antidromic and orthodromic central nervous system (CNS) electrical stimuli will be used to obtain hodological information. The results will guide a more extensive unit analysis in the awake squirrel monkey seated in a restraining chair. Monkeys bar-pressing for food reward will be trained to press a panel, terminating thermal stimuli delivered to either hip; the escape response will interrupt access to food. The force, latency and frequency of escape will monitored for noxious and innocuous thermal stimuli. A computer will be used to control behavioral contingencies and correlate extracellular unit responses with behavioral events. Since there is experimental evidence that apparently selective analgesia can be produced by electrical stimulation of selected CNS sites, such stimuli will be used to further test neural-behavioral correlations during behaviorally identified analgesia.